The invention relates to a method and apparatus for sterilizing containers by introducing heat into the interior of the container.
In a known method, known, for example, from German Offenlegungsschrift No. 28 39 543, a mixture of water vapor and air at a temperature of 250.degree. C. is blown into the interior of the container in order to disinfect it. The heat thus transmitted to the wall of the container can increase the temperature of the wall to such an extent that containers made of plastic are damaged. The attempt is therefore made to attain the desired results using the shortest possible heat-treatment period. The danger here, however, is that an insufficient degree of sterility may result. On the other hand, in containers given a beaker-like form by the thermal deformation of a plastic film, if the heat treatment lasts long enough for sufficient sterilization, re-deformation can occur, because the restoring tensions in the container walls tend to relax with time. This renders the containers unusable, because the container volume is reduced and because the appearance is no longer regular. Excessive heat stress in containers made of multi-layered material can also cause delamination of the layers.
It is accordingly desirable to seek a sterilizing method capable of disinfecting containers to a high degree of sterility, even in the case of thin-walled containers shaped by stretch-molding, in whose walls restoring tensions are present, or of containers made of multi-layered material.
A variant method which avoids performing heat treatments upon the interiors of pre-formed containers, which is disclosed in German Patent No. 23 39 128, is to disinfect the strip of packaging material before it is molded into containers. The degree of sterility attained by this method is quite high; however, it frequently occurs that when the containers are shaped by stretching the strip of packaging material pores will open up on the surface of the material, and these pores can bring about an unsterile state. This occurs especially with strips of packaging material made of mineral-filled or foam-type plastics.